The dehydration tolerance of leaf photochemistry varies greatly along the land plant phylogeny. This study investigated the photochemical response to severe dehydration and subsequent rehydration in 444 species representing the major terrestrial biomes and photosynthetic lineages: lichens, bryophytes, pteridophytes and spermatophytes. Using a standardized, simple, and portable dehydration test, 318 species (84.3% of the total studied) met two key criteria: 1) optimal physiological conditions at the start of the test, and 2) achieving relative water contents (RWC) below 25% at the dehydration stage (with approximately half dropping even further to 5-15%). This ensured all species began under similar conditions and experienced severe tissue dehydration, allowing for subsequent fair comparisons. The maximal photochemical efficiency of PSII (F v/F m) was employed as a physiological indicator at three stages: hydrated (initial), dehydrated and rehydrated. Altogether, lichens and mosses displayed the highest overall F v/F m recovery after dehydration, whereas lower tolerance was observed in liverworts, pteridophytes, gymnosperms, and angiosperms. Four distinct photochemical responses emerged: (i) ”dehydration-sensitive” (low F v/F m during dehydration and after rehydration) (LL) (61.2% of the total species), (ii) ”rehydration-sensitive” (high F v/F m during dehydration, but decreasing after rehydration) (HL) (18.6%), (iii) ”dehydration-tolerant” (low F v/F m during dehydration, high afterwards) (LH) (13.6%) (iv) and ”dehydration-resistant” (high F v/F m throughout) (HH) (6.5%). Pteridophytes and spermatophytes primarily exhibited LL or HL responses. LH species were predominantly found among mosses, lichens, and resurrection ferns, which displayed dehydration-induced reversible photochemical quenching as a major photoprotection strategy. Instead, the HH response showed a balanced distribution across phylogenetic groups but being the less frequent response type and included some angiosperms from polar regions and high-altitude mountain ranges as Colobanthus quitensis and Deschampsia antarctica from Antarctica and Pozoa coriacea from the Andean range. These findings highlight the evolutionary divergence of dehydration tolerance strategies across the land plant phylogeny and provide insights into the underlying photochemical mechanisms. Moreover, the methodology employed in this study offers a field-based tool to gain preliminary insights into the dehydration tolerance of almost any plant.

A trade-off between allocated resources for photosynthesis and stress tolerance is generally observed in nature. Thus, the search for outlier species breaking this trend is an interesting approach to identity new mechanisms for plant breeding purposes. Hypothetically, outlier extremophyte species present a distinctive arrangement of physiological functions that favor stress tolerance mechanisms without jeopardizing investment allocation into photosynthesis. We explored this trade-off, analyzing twenty-one plant species for desiccation tolerance, and photosynthetic capacity, under the extreme arid environments of the Atacama Desert and the Surire Salar in the Chilean Altiplano. Most of the studied species followed the trade-off tendency, however, we did find one outlier species, Prosopis tamarugo. To study the mechanisms involved in this atypical response, the Prosopis genus was analyzed more deeply. Our results suggest that the outlier response of P. tamarugo is multifactorial. This species presented a high photochemistry activity, associated with a higher synthesis of chlorophylls, photoprotective pigments, and complex antioxidant molecules. Moreover, the synthesis of no-nitrogen osmoprotectant molecules, such as ciceritol and mannitol in P. tamarugo, would allow the allocation of nitrogen to support its high photosynthetic capacity, without compromising its leaf desiccation stress tolerance.

Recent results suggest that metabolism-mediated stomatal closure mechanisms are important to regulate differentially the stomatal speediness between ferns and angiosperms. However, evidence directly linking mesophyll metabolism and the slower stomatal conductance (gs) in ferns is missing. Here we investigated the effect of exogenous application of abscisic acid (ABA), sucrose and mannitol on gs kinetics and carried out a metabolic fingerprinting analysis of ferns and angiosperms leaves harvested throughout a diel course. Ferns stomata did not respond to ABA in the time period analysed. No differences in the relative decrease in gs was observed between ferns and the angiosperm following provision of sucrose or mannitol. However, ferns have slower gs responses to these compounds than angiosperms. Metabolomics analysis highlights that ferns have higher accumulation of secondary rather than primary metabolites throughout the diel course, with the opposite being observed in angiosperms. Our results indicate that metabolism-mediated stomatal closure mechanism is conserved among ferns and angiosperms and that the slower stomatal closure in ferns is associated to a reduced capacity to respond to mesophyll-derived sucrose and to a higher carbon allocation toward secondary metabolism, which likely modulates both photosynthesis-stomatal movements and growth-stress tolerance trade-offs.